Vicki Davis offers this rubric for a wiki project. She gives 35 points for content (assessed by a separate tool) and 65 points for the collaboration and the wiki construction. Note that at the bottom of the rubric she states, "Attendees of K12 online conference are given permission to duplicate for classroom use only." Please honor that and not photocopy it for your class. But feel free to be inspired by its content.
Set clear objectives for what content you want your students to include in their wiki as well as how the wiki itself.